The contribution marks the first time the two top-spending Democratic super PACs during the 2012 elections have ever given money to one another, although they’ve forged a close relationship, including co-hosting a major fundraising event at the 2012 Democratic National Convention.

It’s likewise notable because super PACs, which first sprang into existence during 2010, rarely share money with one another, instead fundraising primarily from individuals, and in some cases, unions, for-profit corporations and nonprofit groups.

Most of this money went toward advertisements opposing Republican Gabriel Gomez in Massachusetts’ U.S. Senate special election. Rep. Ed Markey, D-Mass., defeated Gomez in June. It also this week made a $270,000 expenditure against Senate Minority Leader Mitch McConnell, R-Ky., an FEC filing shows.

From the beginning of the year through mid-July, Senate Majority PAC brought in more than $3 million, and has about $980,000 left in its coffers.

The group’s largest contribution from an individual during this period came from DreamWorks CEO Jeffrey Katzenberg, a major Democratic donor who gave $100,000 in June.

Senate Majority PAC also received donations this year from two politically active liberal nonprofit groups: $35,000 from American Bridge 21st Century and just less than $59,000 from Patriot Majority USA.

During the 2012 campaign, Senate Majority PAC spent more than $37 million in support of Democratic Senate hopefuls, according to federal records.

Priorities USA Action has yet to file its mid-year financial report with the FEC — filings are due by July 31 — but the group reported having more than $3.7 million cash on hand at the end of 2012.

The super PAC was founded by former Obama advisers Bill Burton and Sean Sweeney in 2011 to aid the president’s re-election bid, and spent more than $75 million doing so.

The group’s plans for the 2014 and 2016 elections, however, remain unclear, as its leadership has largely scattered.

Since the 2012 race, Burton joined the Washington, D.C.-based public affairs consulting firm Global Strategy Group. Spokeswoman Marcy Stech in February became EMILY’s List’s national press secretary. Compliance and Operations Director Megan Brengarth left in January to join Washington, D.C.-based firm Capitol Compliance Associates.

Representatives for Priorities USA Action could not be immediately reached for comment.

Primary Source dove into developments in the post-Citizens United world of money in politics. It featured original, entrepreneurial reporting by Center staff, as well as examinations of primary source documents.